This patent relates to capacitive transducers, and more particularly to techniques for overcoming electromagnetic interference in capacitive sensors.
In inertial sensors, electromagnetic disturbance or interference (EMI) occurs primarily due to capacitive coupling between bond wires and nearby cables, plates, circuitry, etc. FIG. 1 illustrates an exemplary scenario of EMI. In FIG. 1, a microelectromechanical structure (MEMS) device 102 is coupled to an application specific integrated circuit (ASIC) 104 by a plurality of bond wires 106. A source of EMI 110 that is near the bond wires 106 creates capacitive coupling 112 between the EMI source 110 and the bond wires 106. Capacitor symbols are shown in FIG. 1 to illustrate the capacitive coupling 112, but this simply illustrates the parasitic capacitance between the electromagnetic disturbance source 110 and the bond wires 106, no actual electrical component is present. The bond wires coupling capacitive nodes are the most sensitive to EMI, as opposed to nodes driven by a voltage source or amplifier.
In environments with a high density of electronics, there can be numerous sources of EMI, and these EMI sources can be significant. The electromagnetic disturbances can also occur at substantially a single frequency, which upon sampling can get folded into a DC component. These electromagnetic disturbances can land on top of a desired sensor signal and obliterate the desired signal. For example, if a desired signal is sampled at a 100 kHz clock frequency, and the disturbance is at 100 kHz, then when sampling the disturbance at the clock frequency it can appear as a substantially DC signal. Thus, it is important to protect desired sensor signals, especially along capacitive paths, from EMI. The EMI problem is especially important to solve in safety critical applications that are in harsh environments, for example the sensors used for electronic stability in an automobile.
Two commonly used solutions to EMI are shielding the sensor with metal, and using a differential approach. Shielding the sensor with metal includes creating a Faraday cage to block external electric fields which can cause EMI. However, shielding can be bulky and expensive, especially when there are numerous sensors to be shielded.
The differential approach takes the differences between signals on parallel wires which can substantially subtract out the electromagnetic disturbance as a common mode signal. FIG. 2 illustrates the differential approach with an exemplary differential sensor and amplifier system 200 that includes a MEMS device 220 coupled to an ASIC 240 by bond wires 260, 262. Each of the bond wires 260, 262 experiences EMI from external EMI sources 210. Capacitive coupling 250 between the EMI source 210 and the first bond wire 260 creates a first disturbance capacitance C1, and capacitive coupling 252 between the EMI source 210 and the second bond wire 262 creates a second disturbance capacitance C2. If the disturbance capacitances C1 and C2 between the EMI sources 210 and the bond wires 260, 262 are the same, then the electromagnetic disturbance can be rejected due to the common mode rejection of the differential amplifier of the ASIC 240. However, in order to achieve the desired cancellation, the disturbance capacitances C1 and C2 between the EMI sources 210 and the bond wires 260, 262 should be closely matched, for example a difference of less than 0.5%. This matching can be very difficult to achieve in practice. Even if the matching is achieved initially, bond wires can be disturbed or warped, for example by an automobile accident. This movement of the bond wires can cause asymmetry between the bond wires, which can cause an unwanted mismatch in the disturbance capacitances and reduce the effectiveness of the differential approach. For this reason additional techniques can be used to smear the EMI energy over a wide frequency range.
Accelerometers are often implemented in harsh vibration-ridden environments, for example automotive or industrial environments. In these environments, it is desirable to have accelerometers with good linearity, low drift performance and large full scale range. Self-balanced accelerometers are usually chosen for these applications. In self-balanced accelerometers, the capacitance C is proportional to 1/d, where d is the distance between the capacitive plates; and the measured output voltage V0 is proportional to (C1−C2)/(C1+C2). Combining these two relationships provides:
                              Vo          ∝                                                    C                1                            -                              C                2                                                                    C                1                            +                              C                2                                                    =                                                            1                                  d                  ⁢                                                                          ⁢                  1                                            -                              1                                  d                  ⁢                                                                          ⁢                  2                                                                                    1                                  d                  ⁢                                                                          ⁢                  1                                            +                              1                                  d                  ⁢                                                                          ⁢                  2                                                              =                                                                      d                  ⁢                                                                          ⁢                  2                                -                                  d                  ⁢                                                                          ⁢                  1                                                                              d                  ⁢                                                                          ⁢                  2                                +                                  d                  ⁢                                                                          ⁢                  1                                                      =                          x                              d                ⁢                                                                  ⁢                0                                                                        (        1        )            where x is the displacement value, d0 is the zero displacement value, d1=d0−x is the distance between the plates of capacitor C1, and d2=d0+x is the distance between the plates of capacitor C2. Equation (1) shows that in the ideal case the output voltage V0 of the self-balanced accelerometer is a linear function of the displacement x. Unfortunately, in actual implementations, there are sources of non-linearity not taken into account in Eq. (1).
Though there are several ways to build self-balanced accelerometers to obtain a reading that is proportional to the displacement of the proof mass, to achieve a highly linear accelerometer it is desirable to have a topology that results in zero residual force upon the application of sensor excitation voltages. There are two main sources of non-linearity in self-balanced accelerometers: feed-through capacitance, and mismatch between the two sensor cores. The dominant source is feed-through capacitance, and it is present in both single ended (using only one core) and differential (using two cores) topologies. Feed-through capacitance (Cft) is any fixed capacitance between the proof mass and the sense electrodes. The feed-through capacitances Cft arise due to parasitics in the sensor element and due to capacitance between the bond wires.
To achieve robustness to EMI and spurious vibration, a fully differential accelerometer is typically used for automotive applications. A fully differential self balanced accelerometer for first-order EMI reduction typically has two capacitive cores as described below with reference to FIG. 3. However, two capacitive cores on a MEMS device coupled by bond wires to an integrated circuit requires numerous bonding pads and bond wires, which requires a relatively large area just for connections. It would be desirable to reduce the number of bonding pads and bond wires to reduce the area needed for connections.
It would be desirable to have a robust technique for reducing electromagnetic interference that also overcomes some of the disadvantages of shielding and differential circuits with reduced connections to reduce the area needed for connections. It would also be desirable to reduce or eliminate the nonlinearity due to feed-through capacitances.